The invention relates generally to rolls and, more particularly, to improved temperature control in a hydrostatically supported roll having hydrostatic elements supported by the crosshead that are forced by a pressurized liquid against the inner circumference of the hollow roll for supporting same.
A hydrostatically supported roll is disclosed in Swiss Patent No. 577598. In this patent the support elements are formed as support plungers or pistons guided in cylinder bores disposed in the side of the crosshead facing the roll gap. Choke holes connect support pockets formed on the side of the support plungers facing the inner circumference of the hollow roll with the pressure chamber formed in the cylinder bore below the support plunger. Thus, the pressure chamber and the support pockets are connected to each Other and pressurized liquid passes from the pressure chamber of each support plunger into the support pockets, of which several usually are provided for stabilization. A small amount of pressurized liquid flows over the edge of the support pockets and forms a supporting liquid film such that the support plungers act against the inner circumference of the hollow roll free of metallic friction. A single row of support plungers can be provided, which are supplied with pressurized liquid either individually and independently of each other, in groups or jointly. However, if required, several rows of support plungers can be provided in one roll with the rows being relatively offset from each other in the circumferential direction.
In the roll disclosed in Swiss Patent No. 577598 liquid leaking from the support plungers collects in the space between the inner circumference of the hollow roll and the crosshead, after which it is conducted to a heat exchanger and then to a supply tank. A pump feeds the collected liquid under pressure back to the individual pressure chambers under the support plungers. With this known construction, different pressures maybe fed to individual support elements or to groups of support elements and the line pressure profile generated by the hydrostatic support elements can be adjusted. Due to heat transfer between the pressurized liquid heated or cooled by the heat exchanger and the inner circumference of the hollow roll, the temperature appearing at the outer circumference of the hollow roll is uniform over the width of the web in rolls of the type described above, i.e., rolls in which the heat exchanger constitutes the only temperature adjusting device and is common to all of the support plungers.
If in such a roll the line pressure distribution must be adjusted and the pressure of the pressurized liquid at a given support element accordingly must be increased, for instance, more pressurized liquid flows over the edge of the support element than before the adjustment. Since, in such a roll, the heat transfer depends heavily on the amount of liquid flowing over the edge of the support element at the inner circumference of the hollow roll, more heat is transferred to the inner circumference of the hollow roll in the vicinity of this support element and the temperature distribution along the hollow roll thereby is changed.
One problem in this type of roll is that the pressure and temperature cannot be adjusted independently of each other at a given support element. A change in the line pressure distribution always is accompanied by a corresponding change in the temperature distribution.
FIG. 5 of Swiss Patent No. 577598 shows an embodiment that attempted to correct this shortcoming by providing individual supplementary heaters in the feedline to the individual support elements or to the groups of support elements, in addition to the heat exchanger. In this manner, the temperature achieved by the heat exchanger could be modified. For more stringent temperature operating requirements, this solution is unsatisfactory because the supplemental heaters only provide very sluggish control of temperature conditions. In addition, the quantity of leakage liquid flowing at the hydrostatic element changes upon a change in viscosity caused by a temperature change at the same pressure and, therefore, an change in the pressure conditions occurs. Therefore, actual independence of the temperature and pressure distributions is not achieved.
In German Patent No. 2902956, the hollow roll is temperature-controlled via temperature-controlled pressurized liquid escaping at the support plungers. Supplemental heating is provided, which can be separately controlled in sections. The temperature-controlled pressurized liquid is sprayed against the inner circumference of the hollow roll.
In European Patent No. 201783, heat is transmitted to the hollow roll via temperature control of the pressurized liquid. To change the temperature distribution, the quantity of pressurized liquid transported at a given support plunger is adjusted, thereby adjusting the pressure exerted at this point. In order to compensate for the attendant change in the line pressure distribution, a further row of support elements is provided on the opposite side of the crosshead to automatically equalize the change in the line pressure distribution that occurred. In this manner, a change in the temperature of the roll surface can be achieved without changing the temperature of the pressurized liquid.
In German Patent No. 2503051, the support elements do not provide any temperature control function. This function is accomplished by a row of support elements formed on the opposite side of the crosshead, similar to heat transfer elements, which receive a temperature-controlled pressurized liquid.
In all of the rolls of last three mentioned patents, additional structural elements must be accommodated in the already cramped space in the interior of the hollow roll. When these elements are mounted in holes in the crosshead, provision of the additional elements undesirably weakens the crosshead. Furthermore, the additional cost brought about by providing these elements is a considerable disadvantage.